Plans to pump vast quantities of iron into oceans in a bid to reduce carbon dioxide levels in the atmosphere could trigger a global ecological disaster, scientists warn.

They say that a project backed by US businessmen and researchers to seed the seas with iron could lead to the uncontrollable spread of toxic algae and the release of gases that could damage Earth's fragile ozone layer.

Professor Sallie Chisholm, a marine biologist at Massachusetts Institute of Technology, told The Observer: 'It would be folly on a global scale.' She was backed by Prof Andrew Watson, of East Anglia University: 'It is not just that this project may be dangerous, it is also unethical,' he said. 'What right has one group or country to use the world's oceans to resolve its domestic problems?'

The scheme is based on the discovery that tiny amounts of iron - around one part per billion - are critical in stimulating phytoplankton growth in seas. As such plankton absorbs carbon dioxide, proponents argue that increased iron and plankton levels would lead to the removal of carbon dioxide from the atmosphere, thus helping to slow down global warming.

In many parts of the world, iron in seawater is virtually non-existent - and plankton levels correspondingly low. As a result, several groups of US entrepreneurs have begun experiments aimed at correcting this problem: by pumping tonnes of soluble iron compounds into sea areas.

For example, Planktos, backed by the rock star Neil Young, who provided his yacht Ragland free of charge, has completed trials off Hawaii. Tonnes of iron com pounds were dumped overboard and the growth of plankton measured.

Results have encouraged Planktos - which is funded by donations from US energy companies - and other groups, such as GreenSea Ventures, to consider selling their services.

They hope to charge the US government around $10 for each tonne of carbon that they remove from the atmosphere by pumping iron sulphate solutions into the sea. This would then allow America to reduce the carbon emission cutbacks that it will have to make when it signs the Kyoto agreement on global warming.

In other words, by interfering with the world's oceans, the US will be able to maintain its high output of industrial gases. 'There is no law or international agreement that prevents us from doing this,' said Dr Lee Rice, of GreenSea Venture. 'It is perfectly legal.'

Rice envisages tracts of sea being seeded with soluble iron compounds. Plankton would bloom and then die, sinking to the seabed, carrying with them the carbon dioxide they had absorbed. 'We would only do our seeding episodically - say for 30-day periods - so as not to trigger other undue environmental side effects. Then we would measure how much plankton growth we have stimulated and calculate how much carbon dioxide will have been absorbed. We will then charge for our services.'

But this concept was derided by Chisholm. 'You can certainly measure how much plankton growth you have stimulated - but that will not tell you how much carbon will have sunk to the seabed. Much of it could simply be released back into the atmosphere.'

But if Planktos and GreenSea make money, other groups would be encouraged to carry out similar projects. Vast portions of ocean could be seeded, with dire environmental consequences, as was revealed by biologists from the Moss Landing Marine Laboratories, in California.

They reported that iron-fertilised plankton blooms also produce emissions of methyl bromide which damages the ozone layer, and isoprene - a gas that actually increases the greenhouse effect.

The trouble, say critics, is that studies by Planktos and GreenSea are not sophisticated enough to pick up the environmental implications of their projects. As a result, major programmes could be well under way before it was realised that an ecological disaster had been triggered.

'These companies are gaining momentum at an extraordinary rate. So, yes, we do have a lot to be concerned about,' Chisholm said.

Or as an editorial in Nature warned last week: 'Politicians seem to have been deaf to warnings, leaving organisations like Planktos and GreenSea to pursue their experiments in climate engineering.'